Endovascular Repair of Extracranial Cerebrovascular Lesions

Patient Selection for Carotid Angioplasty and Stenting

Appropriate patient selection for CAS is paramount to a successful procedure, and in minimizing perioperative complications and stroke risk. Part of the motivation in the development of CAS was to address the needs of patients at high risk for CEA in an attempt to improve results of carotid revascularization. In the United States, the Centers for Medicare and Medicaid Services (CMS) has established criteria for patient eligibility for endovascular carotid interventions on the basis of being high risk for CEA, identifying both physiologic and anatomic factors making the patient a high-risk candidate for an open surgical procedure ( Box 21.1 ). Under the current national coverage determination (NCD) policy, CMS coverage for CAS is provided in symptomatic patients with carotid artery stenosis ≥70% or for high-risk patients with symptomatic stenosis ≥50% and asymptomatic stenosis ≥80% who are enrolled in Category B Investigations Device Exemption (IDE) trials or postapproval registries. The NCD requires the use of US Food and Drug Administration (FDA)-approved CAS systems and embolic protection devices, as well as CMS-approved institutions. The use of CAS for patients at high risk for CEA is now an established practice; however, CMS coverage has a substantial impact on patient selection and may not have equipoise with evolving techniques and approaches to CAS.

There are also factors that make patients high risk for CAS, which are primarily anatomic. Examples of poor anatomy for CAS include severe tortuosity of the aortic arch, great vessels, or carotid bifurcation, heavy calcification of these vessels, or unsuitable access. Marked angulation, kinks, and coils of the internal carotid artery (ICA) might not allow adequate room for appropriate deployment of cerebral protection devices (CPDs) with adequate wall apposition. If these areas are straightened by stents, the angulation is usually displaced distally and may be more exaggerated. Difficult arch anatomy or severe calcification of the aortic arch and proximal branch vessels may preclude a safe transfemoral carotid intervention. Carotid bifurcation lesions that are recently symptomatic or are composed of soft plaque should also be avoided, if possible. These lesions also can be treated using proximal protection, so that the cerebral circulation is protected from emboli before crossing the lesion. Carotid bifurcation lesions that are too long to treat with a single stent also tend to add risk to the CAS procedure. Poor early results of CAS in older patients have prompted a high level of caution in octogenarians. A recent retrospective review of 135 octogenarians undergoing CAS reported a peri-interventional complication rate of 8.9%, with a low rate of neurological events over 33 months of follow-up. However, a high total death rate was reported, which may be due to the specific patient cohort and comorbidities and not necessarily procedurally related. Extra evaluation for high-risk anatomy, preexisting brain lesions, evidence of cognitive problems, or other factors that may make repair more risky or limit its long-term value should be performed. In the early phases of CAS, these high-risk factors for performing CAS were not yet established. It is now recognized that CAS in its current form and with existing technology is not a direct replacement product for CEA. Nevertheless, the results of CAS have improved steadily over the past 15 years. One of the reasons for this is improved patient selection based on a better understanding of which patients are at high risk for CAS ( Table 21.1 ).

CPDs have also evolved over the last decade from distal occlusion balloons to distal filters and, subsequently, the addition of proximal balloon occlusion or clamp occlusion and flow reversal techniques. Distal filters have replaced distal occlusion balloons that were cumbersome to deploy and compromised visualization of the ICA by occluding flow, and risks of ICA dissection. Distal filter protection requires arch manipulation and crossing the lesion prior to filter placement. Furthermore, unprotected balloon dilation may be required to cross the lesion and place the filter. Thus cerebral embolization may occur before cerebral protection is established. Distal filters may not provide complete protection due to pore size, or lack of wall apposition, and they may become filled with debris, increasing the risk of embolization during retrieval. Distal protection systems should not be used in distal ICA tortuosity, which may lead to inadequate vessel wall apposition and retrieval difficulty, or if there is an inadequate landing zone for the device. Proximal protection devices are useful for complex lesions, tortuous anatomy, or a symptomatic or ulcerated plaque that is at higher risk for distal embolization. Proximal occlusion devices stop or reverse flow in the ICA and eliminate crossing the lesion prior to embolic protection. The Medtronic MoMa proximal protection device uses balloon occlusion of the common carotid artery (CCA) and external carotid artery (ECA) to stop flow. The bifurcation is aspirated after stent placement. However, this system requires initial catheterization of the arch and great vessel origins as well as ECA occlusion prior to the establishment of protection, which both pose embolic risks. The direct trancervical approach using the ENROUTE transcarotid neuroprotection system (NPS) is another approach to CAS, which is designed to avoid the risk of embolization during unprotected catheterization of the aortic arch and supra-aortic vessels from a transfemoral approach. In this approach, the CCA is exposed through a short supraclavicular incision and cannulated directly. This requires adequate CCA length (a minimum of 6 cm from clavicle to bifurcation) and minimal atherosclerotic disease in the CCA to permit safe sheath placement and arterial clamping. A reversed-flow circuit is established between the CCA sheath and a femoral vein cannula. The minimal supraclavicular incision required for this approach remains a viable option in patients who are poor candidates for standard CEA due to poor arch anatomy, hostile neck anatomy, or medical unsuitability.

The patient must be neurologically intact and able to follow commands to permit safe carotid stenting. Patients who cannot lie flat on the fluoroscopy table because of shortness of breath from cardiac or pulmonary problems will not tolerate intervention. Morbid obesity makes femoral access more challenging, as well as control of the access site after the procedure. Patients who cannot be treated safely with antiplatelet agents before and after the procedure, or who are at high risk for hemorrhagic complications, should not undergo endovascular carotid interventions.

Preprocedure Evaluation

Thorough evaluation and preparation of the patient before the procedure is essential for safe carotid intervention. The brachiocephalic anatomy is studied before the procedure to assess candidacy for the percutaneous approach. A thorough understanding of the arch, carotid, and cerebral arterial anatomy can be obtained with catheter-based arteriogram, computed tomographic angiogram (CTA), or magnetic resonance angiography (MRA). CTA is advantageous for multiple reasons. It is fast, depicts the arch anatomy well with good vessel wall and lumen definition, identifies calcifications, and can be used to measure the distance from the clavicle to the carotid bifurcation, which is important for the transcervical approach. MRA offers anatomical detail without radiation exposure, but it is slow and may be uncomfortable for some patients. Both CTA and MRA provide information on plaque morphology. Several anatomic factors can be considered relative contraindications to CAS, as mentioned previously, and these are well identified using preoperative imaging.

A National Institutes of Health (NIH) stroke scale or other objective evaluation is completed before to CAS. A CT or magnetic resonance image (MRI) of the brain is obtained in symptomatic patients and in those 80 years of age or older to evaluate for preprocedure cerebral pathology. Octogenarians have a higher risk of stroke with CAS; therefore, it should be performed with caution. Antiplatelet therapy is administered: aspirin daily, and clopidogrel (Plavix) 75 mg/day for 5 days before the procedure. In all cases, patients should have received clopidogrel (total dose of 300 mg) before the intervention. Antihypertensive medication is held or decreased on the day of the procedure. If an antihypertensive is required during the procedure, it is best to use a short-acting agent. Postoperative hypotension or bradycardia can occur after CAS as a result of baroreceptor stimulation. Patients with aortic stenosis may have cardiovascular collapse in this setting, and external pacing pads or a temporary internal pacemaker should be placed. In patients with absent femoral pulses owing to aortoiliac occlusion or hostile groin anatomy, a transbrachial or transcervical approach may be considered.

The CAS procedure is performed using local anesthesia with minimal or no sedation to facilitate patient cooperation and continuous neurologic monitoring. Continuous arterial pressure monitoring is required. Techniques, such as squeezing a rubber toy, aid in simple and effective neurologic monitoring during the procedure.

Approved carotid stenting systems are limited to use in patients with symptomatic ≥50% stenosis, or asymptomatic ≥80% stenosis ( Table 21.2 ). CAS in standard risk patients and those at high risk for CEA but who are asymptomatic are not currently approved for reimbursement under Medicare guidelines, unless it is performed as part of an approved clinical research protocol. Reconsideration of the extent of coverage of CAS by the CMS is likely to take place as a result of recent data accumulated through recent RCTs.

Access for Transfemoral Approach

Femoral artery access is the usual approach when planning to use a distal filter or proximal balloon occlusion for cerebral protection. The majority of CAS cases have been performed through femoral artery access. The right common femoral approach is the most convenient for catheter manipulations by the right-handed surgeon, but the left is acceptable as well if the right side is hostile. A micropuncture set (21-gauge needle) can be used for the initial femoral access; this has significantly reduced the number of femoral access complications. The use of ultrasound during puncture has become routine, and it has enhanced the ability to use closure devices at the conclusion of the procedure. Following guidewire access, an introducer sheath is placed in the common femoral artery, which is the same size as that intended for the carotid stent placement (typically 6 or 7 French) or CPD (8 or 9 French for the MoMa device). If a brachial approach is planned, access to the CCA is usually best obtained from the contralateral brachial artery. Transcervical access will also be discussed later in this chapter.

Arch Evaluation

Arch manipulation carries a risk of neurologic events. In several studies of CAS, up to 1% of patients sustained a stroke in the contralateral hemisphere, suggesting that carotid access is a contributor to morbidity. Systemic heparin should be administered before aortic arch manipulation. An activated clotting time (ACT) of 250 seconds or greater is desired.

Following systemic anticoagulation, an arch aortogram is performed using a multi-sidehole flush catheter (e.g., pigtail catheter) with the image intensifier in a left anterior oblique (LAO) position. The goal is to obtain an en face view of the aortic arch as it traverses posterior and laterally toward the patient's left. An angle of 30 to 45 degrees LAO usually provides an optimal view of the origins of the arch vessels. The pigtail catheter is subsequently withdrawn over a 260-cm angled Glidewire. The guidewire and pigtail catheter should be pulled back together into the descending aorta where the pigtail can then be removed and the guidewire can be advanced into the ascending aorta. As few manipulations as possible are performed in the aortic arch and great vessels to lower the risk of an iatrogenic embolic event.

Hypertension and advanced age are associated with increased tortuosity of the access pathway to the carotid bifurcation. This makes no difference in the performance of CEA, but directly influences the challenges posed for transfemoral CAS. Negotiating the tortuous arch requires more manipulation for catheterization, a more embedded position of the exchange guidewire, and more maneuvers to achieve sheath placement. The tortuosity of the arch can be assessed rapidly by drawing a horizontal line across the apex of the inner curvature of the arch. Vessels that originate below the horizontal line at the apex of the aortic arch (e.g., branches that arise from the ascending aorta) are more difficult to selectively cannulate ( Fig. 21.1 ). The authors caution against transfemoral carotid stenting in the setting of a “difficult arch” until the operator has become expert with selective cannulation of the common carotid arteries in this situation. Even then, the tortuous arch likely poses a slight increase in the overall risk of CAS. Training and credentialing documents suggest varying numbers of carotid arteriograms as a prerequisite to initiating CAS training.